Seismic exploration for exploring submarine natural resources such as petroleum, natural gas, gas hydrate, or the like, or bedrock investigation for marine construction such as undersea pipeline and cable burying, undersea tunnel, undersea storage equipment, bridge, or the like, has been performed. High-resolution marine seismic survey for the purpose of engineering such as marine engineering and construction works of the submarine pipeline, the cable burying, the submarine tunnel, the submarine storage equipment, the bridge, etc., is to understand a shallow geological structure in which a water depth is shallow.
The high-resolution marine seismic exploration needs precise exploration so that the vertical resolution of exploration data is about 1 m. However, a swell or a wave on the sea which always happens by an ocean current, a tidal current, and a wind affects the seismic data and thus it is difficult to precisely figure out a sub-bottom structure on the seismic sections. The swell effect on the seismic sections is represented by sea-bottom trembling in a saw-toothed shape. This phenomenon affects the whole seismic sections to reduce continuity of the seismic events related to the sub-bottom structures.
As a general data acquisition system of high-resolution marine seismic survey, seismic source generators such as a small air-gun, a sparker, and a boomer, and a receiver of a multi-channel streamer of a single channel or 8 channels and 24 channels or less, etc., have been mainly used. Alternatively, the exploration may also be performed by using a chirp sub-bottom profiler (hereinafter, referred to as ‘chirp SBP’). These explorations are to understand the shallow geological structure and therefore use a relatively higher frequency to be able to acquire seismic data having a low penetration depth but high resolution. When the seismic source is the small air-gun, frequency bands correspond to 100 to 350 Hz, when the seismic source is the sparker, frequency bands correspond to 600 to 900 Hz, when the seismic source is the boomer, frequency bands correspond to 1000 to 2000 Hz, and when the seismic source is the chirp SBP, frequency bands correspond to 2000 to 7000 Hz.
Meanwhile, a vertical resolution may be represented by the following Equation 1.
                              R          V                =                                            1              4                        ⁢            λ                    =                      v                          4              ⁢              f                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In the above Equation 1, λ represents a wavelength, v represents a medium velocity, and f represents a frequency. For example, when a velocity of marine shallow sedimentary layer is 1500 m/s, the vertical resolution of the air-gun is about 1.07 to 3.75 m, the vertical resolution of the sparker is about 0.42 to 0.63 m, the vertical resolution of the boomer is about 0.19 to 0.38 m, and the vertical resolution of the chirp SBP is about 0.054 to 0.188 m.
Generally, in the case of the high-resolution marine seismic survey, the seismic survey is performed by equidistance shooting, not equidistance shooting and a time interval is determined depending on a water depth of the survey area and power of the seismic source. In the exploration using the sparker, the boomer, and the chirp SBP, the shot interval is made every 1 second at a water depth of 100 m or less. Converting it into a distance, the shooting is made once every about 2.57 m when a velocity of a survey vessel is about 5 knots. A vertical axis of the seismic section is represented by two-way travel time and a horizontal axis is represented by the number of shots. When the exploration is made under the marine environments that the swell having a size of 1 to 2 m is present, the survey is affected by the swell effect every 2.57 m at the horizontal axis and at the same time the travel time at the vertical axis moves up or down every 0.676 to 1.351 ms (seawater velocity=1,480 m/s).
As a frequency of the seismic source is getting higher, the vertical resolution is increased but the seismic source is more sensitive to the effect of swell and wave. Further, under conditions of a shallow marine, the shot interval becomes narrower. For this reason, even in this case, the effect of the swell and the wave is recorded in the seismic data. Therefore, the high-resolution seismic data may be more affected by the swell and the wave.
For example, if the marine seismic survey is performed on four main survey lines of south-north direction and two auxiliary survey lines of east-west direction, a total of eight intersection points are generated. Since each intersection point passes through the same point, the two-way travel time needs to be same. However, the wind, tidal current, and ocean current directions for the same survey line are changed on the sea depending on a south-north direction or a north-south direction, such that the difference in the two-way travel time may occur. Therefore, the difference in the two-way travel time occurs every intersection point. This phenomenon causes an error upon manufacturing an isopach map, a time structure map, etc., when the number of intersection points is increased. Further, as the frequency of the seismic source is getting higher, the wavelength becomes shorter. Therefore, the intersection point error more clearly appears in the high-resolution seismic data.
Therefore, to acquire the high-resolution seismic data, the correction method with more improved accuracy is required.